1. Field of the Invention
The present invention relates to analog signal mixers, and in particular, to analog signal mixers in which the harmonic content of the output signal is limited.
2. Description of the Related Art
An important component used in virtually all wireless transmission and reception systems is an analog signal mixer. A mixer is variously used to modulate, demodulate and frequency-translate signals. A mixer can be used to up-convert a signal, i.e. translate a signal's frequency up, with or without modulation, and also to down-convert i.e. translate a signal's frequency down, with or without demodulation.
Referring to FIG. 1, a common conventional mixer configuration is that of a multiplier mixer, which can be realized in a number of different ways. A multiplier mixer typically receives a local oscillator ("LO") signal V.sub.1 and a radio frequency ("RF") signal V.sub.2, and multiplies them to produce an output signal V.sub.0. The magnitude and frequency of the output signal V.sub.0 are dependent upon the respective magnitudes and frequencies of the input signals V.sub.1 and V.sub.2. This dependence can be represented as follows: ##EQU1## where: V.sub.0 =output signal voltage
.vertline.V.sub.1 .vertline.=magnitude of carrier signal voltage PA1 f.sub.1 =frequency of carrier signal in Hertz PA1 .vertline.V.sub.2 .vertline.=magnitude of modulating signal voltage PA1 f.sub.2 =frequency of modulating signal in Hertz PA1 t=time in seconds PA1 cos[x]=cosine function of "x" PA1 V.sub.0 =output signal voltage PA1 I.sub.EE =emitters' dc supply current in amperes PA1 R.sub.C =collectors' output resistor in ohms PA1 V.sub.1 =.vertline.V.sub.1 .vertline..cos(2.pi.f.sub.1 t) PA1 V.sub.T =transistor base-emitter junction forward bias threshold voltage (.apprxeq.25 millivolts) PA1 V.sub.2 =.vertline.V.sub.2 .vertline..cos(2.pi.f.sub.2 t) PA1 tanh[x]=hyperbolic tangent function of "x" PA1 I.sub.C =collector current in amperes PA1 I.sub.S =BJT saturation current in amperes ##EQU3## V.sub.BB =dc supply voltage (to base) K.sub.C =scalar constant, where C {0, 1, 2, . . . } PA1 n!=(n) (n-1) (n-2) . . . (1),
Referring to FIG. 2, one conventional mixer design is that of a Gilbert multiplier, sometimes referred to as a quad mixer. This type of mixer receives a dc bias voltage V.sub.CC via two resistors R.sub.C and a dc bias current I.sub.EE, as shown. The LO signal V.sub.1 is applied differentially to parallel differential amplifiers, each comprising two matched transistors. The RF signal V.sub.2 is applied differentially to another differential amplifier, also comprising two matched transistors as shown. The output signal V.sub.0, taken across the outputs of the parallel differential amplifiers, as shown, is a function of the two input signals V.sub.1 and V.sub.1, as follows: EQU V.sub.0 =I.sub.EE R.sub.C.tanh[V.sub.1 /(2V.sub.T)].tanh[V.sub.2 /(2V.sub.T)]
where:
Advantages of a Gilbert multiplier mixer included conversion gain, direct multiplication of the input signals and good compatibility with monolithic silicon integration techniques. However, disadvantages include "noisy" operation, distortion at signal levels above 2V.sub.T (unless degeneration is used) and poor operation under low voltage conditions, e.g. with V.sub.CC below three volts.
Referring to FIG. 3, another conventional mixer design uses the non-linear device characteristics of a semiconductor such as a bipolar junction transistor ("BJT"). A BJT in a common emitter configuration with collector V.sub.CC and base V.sub.BB bias voltages, receives its LO signal V.sub.1 and RF signal V.sub.2, summed together, at its base. Due to the inherent non-linearity of the transistor's operating characteristics, the output voltage V.sub.0 is a function of the input signals V.sub.1 and V.sub.2, as follows: ##EQU2## where: V.sub.CC =dc supply voltage (to collector)
where n {1,2,3, . . . }
Advantages of a "non-linear device" mixer include conversion gain, simplicity of design and good performance with respect to noise. However, a major disadvantage is the generation of undesired frequency terms, or harmonics, namely signal energy at frequency multiples of the mixing products of the input signals (e.g. signal energy at frequency multiples of the input signal frequencies, as well as combinations of multiples of, sums of and differences between the input signal frequencies).
Referring to FIG. 4, another conventional mixer design is a diode ring mixer. A ring of four diodes (or sometimes eight diodes) are connected in a bridge configuration to the center-tapped secondary windings of two transformers, as shown. The LO signal V.sub.1 and RF signal V.sub.2 are applied to the primary windings of the transformers, and the output signal V.sub.0 is taken from the center tap of one of the transformers, as shown. Similar to the non-linear device mixer discussed above, the output signal V.sub.0 is dependent upon the two input signals V.sub.1 and V.sub.2.
Advantages of a diode ring mixer include good performance with respect to noise, and a wide frequency range of use (e.g. up to many gigahertz). However, disadvantages include the need for a highlevel LO signal V.sub.1 and the need for transformers (or hybrid couplers) which thereby renders this design poorly suited to monolithic silicon integration techniques.
Accordingly, it would be desirable to have an improved mixer design which combines more of the advantages with fewer of the disadvantages of the foregoing conventional mixer designs.